A thing we are discussing heavily in my digital imaging class is how to do composite images. We are learning about several different applications where creating a composite image is useful, and one of the major ones is in balancing two light sources – an interior with windows, to be exact. The problem here is that the interior you are photographing will often be a couple of stops darker than the scene seen through the window. To get both properly exposed, two images are captured without moving the camera, only changing shutter speed or ISO. The final result, after some selecting and masking in Photoshop, is a single image that appears properly exposed both outside and inside.